When using separate heads for reading and writing, such as magneto-resistive (MR) heads for reading and inductive heads for writing, it is difficult to position alternately the read head to the track during reading, and the write head to the track during writing. This is not a serious problem when the misalignment between the read head and the write head is much smaller than the overall track misregistration and the track pitch of the system. However, in high track density applications or in systems with rotary actuators, where the read to write misalignment is no longer insignificant when compared to the track pitch, one is faced with two choices: (1) increase the degree of write-wide and read-narrow, i.e., (a) increase the write track width which results in lower track density, or (b) reduce read track width which results in lower signal-to-noise ratio--either of which results in performance degradation; or (2) devise a servo scheme to position the write head on the track for writing and the read head on the track for reading.
One arrangement heretofore proposed to independently position the read and the write heads to the track is described in the October 1978 issue of the IBM Technical Disclosure Bulletin at page 2005 and depicted in FIGS. 1A and 1B. When positioning the head for writing, the write head is used to read the sector servo information, as in FIG. 1A. When positioning the head for reading, the read head is used to read the sector servo information, as in FIG. 1B.
Another arrangement proposed to solve this misalignment problem is described in the June 1974 issue of the IBM Technical Disclosure Bulletin, at page 217. It requires reading a calibration pattern with both the read head and write head, and using that calibration pattern to measure the misalignment between the read and write heads. The misalignment is then stored and used by the head positioning servo system to offset the head position during read to compensate for the misregistration between the read and write elements. This approach requires the write head to read servo type information only during the calibration phase, which can be done in the factory, but requires a write head designed to read the calibration pattern and electronic circuitry that will permit reading with the write head.
In both approaches, the write head not only has to write data but also has to read the servo information. This requirement put significant limitations on applicability of the scheme, especially in high track density applications with MR-read/inductive-write heads. First, inductive heads have lower signal-to-noise ratio; and although this problem can be remedied to some degree with more turns in the coil, it adds process complexity and cost to head fabrication. Second, an inductive head optimized for writing will most likely have difficulty reading data at data frequency; and although servo information can be written at lower data rate, it adds to the real estate taken up for servo. Third, this arrangement requires one additional read channel for the write head. Finally, and also most important, thin film inductive heads with very narrow pole widths are likely to have domain problems for reading purposes, rendering them unusable for high track density applications.
There is a need for a method and means for compensating for misalignment of separate but linked read and write heads when reading and writing data on a magnetic disk whether such misalignment varies due to the arcuate direction of access with a rotary actuator or is of essentially constant magnitude due to variations in the relative positions at which the heads are mounted during manufacture of a radial linear actuator.